Pyroelectric copying device

ABSTRACT

THERE IS DISCLOSED AN ELECTROSTATIC COPYING DEVICE BASED UPON THE PHENOMENON OF PYROELECTRICITY IN SUITABLE MATERIALS SUCH AS POLYMERIC POLYVINYLIDENE FLUORIDE. LOCALIZED TEMPERATURE CHANGES CAUSED BY AN INTENSE LIGHT SOURCE GENRATE ELECTROSTATIC CHRGE PATTERNS ON THE MATERIAL WHEN EXPOSED TO THE IMAGE OF AN OBJECT INTERPOSED BETWEEN THE LIGHT SOURCE AND THE MATERIAL. BY USING THIN PYROELECTRIC FILMS TO MINIMIZE THERMAL DIFFUSION, IT IS POSSIBLE TO OBTAIN SUFFICIENT RESOLUTION TO ALLOW THE CHARGE PATTERNS TO BE OBSERVED AFTER TONING THE FILMS WITH ELECTROSTATICALLY CHARGED INKS. THE DEVICE IS COMPARABLE IN MANY RESPECTES TO CONVENTIONAL XEROGRAPHIC DEVICES AND ADVANTAGES IN THE SENSE THAT ITS RESPONSE IS NOT WAVELENGTH DEPENDENT AND ITS STRUCTURE AND OPERATION ARE CONSIDERABLY SIMPLIFIED.

July 16, 1974 J. G. BERMAN. JR.. ET AL 3,824,098

PYROELECTRIC COPYING DEVICE Filed June 23, 1972 United States Patent O 3,824,098 PYROELECTRIC COPYING DEVICE John George Bergman, Jr., Rumson, and Glen Robert Crane, Scotch Plains, N.J., assignors to Bell Telephone Laboratories, Incorporated, Berkeley Heights, NJ.

Filed June 23, 1972, Ser. No. 265,569 Int. Cl. G03c 5/08 US. C]. 96-15 7 Claims ABSTRACT OF THE DISCLOSURE There is disclosed an electrostatic copying device based upon the phenomenon of pyroelectricity in suitable materials such as polymeric polyvinylidene fluoride. Localized temperature changes caused by an intense light source generate electrostatic charge patterns on the material when exposed to the image of an object interposed between the light source and the material. By using thin pyroelectric films to minimize thermal diffusion, it is possible to obtain sui'ficient resolution to allow the charge patterns to be observed after toning the films with electrostatically charged inks. The device is comparable in many respects to conventional xerographic devices and advantageous in the sense that its response is not wavelength dependent and its structure and operation are considerably simplified.

FIELD OF THE INVENTION This invention is concerned with an arrangement for recording and reproducing images and, more particularly, with such an arrangement based upon the phenomenon of pyroelectricity.

BACKGROUND OF THE INVENTION There have been devised over the past few decades a great variety of arrangements for recording and reproducing images of objects. Xerography, for example, has recently emerged as a relatively broad and highly developed field of technology. Xerographic devices and processes are based upon the phenomenon of photoconductivity, a property exhibited by a class of materials in which their electrical conductivity is changed upon exposure to light. In probably the best-known application of xerography, a thin layer of a highly insulating photoconductive material, such as selenium, is first sensitized by the application of a uniform electrical potential, and then exposed to a light image in the pattern of a document to be copied. Where the light strikes the surface of the layer, the electrical potential decays, leaving a distribution of electrostatic charge thereon corresponding to the image. This electrostatic image is developed, generally by dusting the layer with an electrostatically charged, dry powdered ink, called a toner. The image is then typically transferred and fixed to paper as a permanent copy. The pre-sensitization of the photoconductive layer by the application of a uniform charge is common to essentially all forms of xerography.

Recent developments have also been made in thermography and thermographic recording and copying devices and processes. In one class of thermographic devices, special temperature-sensitive papers are employed which respond, under the influence of localized heating, by becoming tackified or by a change in color. The former type of papers can be developed by adhering colored powders to the tackified areas of the papers. Papers of the latter type are typically impregnated with one or more chemicals which, upon exposure to intense light image, react to the absorbed energy to produce the color change. They may also be prepared by overcoating a dark-colored paper with a light-colored layer which melts upon heating to reveal the undercoating.

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Another class of thermographic copying devices is based upon the localized change in the conductivity or resistivity of certain materials upon exposure to heat. It has been shown, for example, that if an electrically insulating layer of material, such as polyethylene, is first uniformly charged and then exposed to an intense light source through a document to be copied, the charge on the layer will be conducted away more rapidly in the areas of the layer which are heated by the absorbed energy, due to the increase in its conductivity with heat. The image of the document is then represented by an electrostatic charge pattern on the layer which can be developed or toned by any of the standard xerographic means. A modification of this device utilizes a group of materials, the electrical resistivity of which increases on being heated and does not revert to the original value upon cooling. Thus, a layer of such a material can be exposed to a heat image and then subjected to uniform charging. The charge leaks away in the unheated or cool areas of the layer, leaving an electrostatic image which remains upon cooling to room temperature and which can then be xerographically toned.

Although the foregoing copying devices have found considerable commercial success and practical application, some in greater degrees than others, each of the devices has inherent drawbacks. In the xerographic device, for example, the photoresponse of the selenium photoconductor is sharply wavelength dependent, i.e., strong at about 4000 Angstroms (A.) and practically zero for wavelengths longer than about 5500 A. This fact accounts for the well-known difficulty that xerographic devices have in copying blue lines on white paper, since the blue image discharges the electrical potential of the predominantly blue-sensitive selenium almost to the same extent as does the white background on which it appears.

The thermographic devices can be relatively expensive and of limited usefulness since the permanent copy is typically produced on the special temperature-sensitive paper itself. Thus, the thermographic devices are not preferred for general copy Work, such as, for example, of business documents or for other areas where numerous copies of an original are desired. The fact that the copy is on the temperature-sensitive paper also renders the copy subject to deterioration due to handling and'age.

Recent developments in radiation detection have focused attention on a phenomenon which for many years had been a mere laboratory curiosity. This phenomenon, pyroelectricity, is broadly defined as the property of certain materials which results in the movement of electrostatic charge and in the generation of localized voltages in the materials during a period of charging temperature. It has been known for some time that the response of pyroelectric materials is relatively constant over their entire inherent or imposed absorption range of impinging radiation. Accordingly, use has been made of this phenomenon for signal detection over an extensive range of optical wavelengths, including infrared and ultraviolet wavelengths, as well as the visible spectrum.

SUMMARY OF THE INVENTION The present invention is based upon our recognition that the phenomenon of pyroelectricity can be successfully and advantageously utilized in an electrostatic copying device. The arrangement of our invention is similar to the conventional xerographic arrangements with the exception that the photoconductive semiconductor element of the latter is replaced by a suitable pyroelectric element. Our preliminary studies indicate that several basic advantages over the prior art xerographic and thermographic copying devices are possible with our invention. First of all, the pyroelectric unit of the invention generates its own charge and does not require a sensitization by the application of a uniform electrical potential. Accordingly, the process of our invention typically involves at least one step less than the prior art processes; and the device of an invention is substantially simplified due to the lack of the requirement of a charging means for the active layer. Additionally, in contrast to xerographic devices, the response of the pyroelectric unit of our invention is not wavelength dependent. Thus, the sensitivity and resolution of the device is substantially constant over a broad range of wavelengths of illuminating light, including the infrared, visible and ultraviolet regions. Finally, like most xerographic devices and unlike most thermographic devices, the electrostatic image reproduced on the pyroelectric unit of our invention can be readily transferred and fixed to regular white stationary or to some other suitable surface. The pyroelectric unit is, therefore, continuously reusable and numerous permanent copies of an original can be economically produced.

As an illustrative example of the principles of our invention, a simplified embodiment is described in which the pyroelectric unit comprises a thin film or sheet of polymeric polyvinylidene fluoride (PVF The film of PVF is crystallographically aligned and electrically poled so as to exhibit a net dipolar moment in the direction of the film thickness. Deposited on one of the major surfaces of the PVF film is a thin conducting layer of metal, such as aluminum. The surface of the thin conducting layer is in turn coated with a thin light-absorbing layer, such as conventional flat black paint. Th absorbing layer of this unit is exposed to a beam from a intense light source. Localized temperature changes caused by the absorbed light generate an electrostatic charge pattern on the surface of the PVF film in the image of an object or document interposed between the light source and the film. By employing relatively thin films of the pyroelectric material to minimize thermal diffusion, resolutions have been obtained which allow the electrostatic image to be observed by dusting the PVF film with conventional electrostatically charged inks while the film is being exposed. The toner image is transferred by placing an electrostatically charged paper against the toned PVF film. As the paper is peeled away, the ink is thus transferred.

The use of PVF in the pyroelectric unit is only exemplary of a large class of materials useful in our invention. It is only necessary that the materials exhibit primary or secondary pyroelectric properties and that they be capable of fabrication into sheets of the desired size and thickness. A whole class of organic polymer materials including PVF can be employed. Although such polymeric pyroelectric materials are typically less sensitive than inorganic pyroelectrics, they are preferred since they are readily available or cheaply fabricated into sections of the desired large area and small thickness. Other common inorganic materials exhibiting pyroelectric properties such as many ceramics and single crystalline materials can also be used.

BRIEF DESCRIPTION OF THE DRAWING A more complete understanding of the foregoing and other features and advantages of our invention can be obtained from the following detailed description taken in conjunction with the accompanying drawing in which the sole figure is a perspective view, partly in section, of an illustrative embodiment for exposing, developing, and recording an image using the pyroelectric effect in accordance with the invention.

DETAILED DESCRIPTION (1) The Figure Inthe figure, a simplified embodiment of the invention is illustrated for convenience. Light from intense source 11 is focused by lens 12 and directed through document 13 to be copied. The light image from document 13 is then projected by lens 15 on to recording unit 16. In the embodiment shown, unit 16 comprises three layers of material disposed in surface contact. Layer 17, the layer onto which the image is projected, is an absorbing layer of a material, such as conventional flat black paint. This layer serves to minimize reflection of the incident light and, thereby, to maximize the conversion of radiant energy into heat. Layer 18 is a conducting layer of metal, such as aluminum, which is electrically grounded. It has been found that the use of such a thin conducting layer maintained at ground potential enhances the ability of a unit to accept electrostatically charged inks during development. It also gives a suitable reference from which to measure voltages in the system.

Layer 19 of unit 16 is an insulating pyroelectric layer and consists of a thin film or sheet which exhibits primary or secondary pyroelectric properties. Such materials are characterized in that localized heating thereof results in the movement of electrostatic charge and in the generation of localized voltages in the material in proportion to lo calized temperature changes. A more detailed description of the composition and preparation of pyroelectric materials suitable for layer 19 is given hereinafter. Polymeric pyrolectric films have been found to be preferable because of the ease and low cost with which they can be fabricated in any shape or size.

In operation, the impinging light image from document 13 is absorbed by layer 17 to cause localized heating of pyroelectric layer 19 by direct conduction through layer 18. The heated regions on layer 19 correspond to the background adjacent to the characters AB of document 13. The unheated or cool regions in the layer correspond to the characters AB themselves. In this example, the direction of the net dipolar moment of pyroelectric layer 19 is oriented so that the localized heating of the layer gives rise to localized regions of negative charge on surface 21. The relatively cool regions of surface 21 maintain a relative positive charge. The magnitude of the negative charge generated is proportional to the localized intensity of the absorbed light. Accordingly, an electrostatic image in the pattern of the light image from document 13 appears on surface 21 of layer 19.

Any of the standard means for developing electrostatic images may be used to apply toner to surface 21 of locally heated layer 19. For example, as shown in the figure, it is possible simply to pour negatively charged, dry, powdered ink 23 from hopper 25, shown in section, over surface 21 while the unit is being exposed. The dry ink powders are typically transported to surface 21 by a carrier of some inert material, such as glass beads or fine magnetic particles. The negative toner is pulled away from the carrier, attracted to the cool regions of the surface and repelled from the heated regions. Visible toner image 27 of document 13 results on the surface.

Image 27 can then betransferred and fixed to ordinary paper or to some other suitable surface again by the usual xerographic techniques. After layer 19 is toned, a suitable sheet of adhesive paper 28 can be pressed against surface 21. As the paper is peeled away, it can be made to retain a substantial portion of the toner image 27 as a permanent image 29 (e.g., by heating). Alternatively, toner image 27 can be transferred electostatically. A positively charged sheet of paper can be brought in close contact with surface 21 of layer 19. Such a sheet attracts a portion of the negative toner image 27 and can be processed by standard techniques to retain the image permanently. The latter transfer technique would require a suitable charging means for the paper, such as a standard corona discharge wire.

Since pyroelectric layer 19 loses its charge as it cools, unit 16 must be developed or toned during or immediately after exposure of the image. Effective lifetimes for the charge patterns subsequent to shutting source 11 off have been found to be of the order of a second for PVF It is thus preferable that the charge pattern be toned and transferred while source 11 is on. Reasonable time delays between exposure and toning, and toning and transfer may be possible with different pyroelectrics and with modified embodiments of the invention.

The common pyroelectric materials found to be useful in layer 19 of the figure have pyroelectric coefiicients dP/dT of the order of l0- coul./cm. C. One can see, therefore, that a localized temperature rise due to absorbed light image from document 13 of one C. can generate a localized electrostatic charge of the order of coul./cm. This compares favorably with presently available xerographic copiers, based upon the photoconductive effect, which involve localized charges of this order.

It is noted that the essential difference between the devices of our present invention and existing xerographic devices arises in the basic phenomenon responsible for producing the electrostatic image in the pattern of the object to be reproduced. The similarity in other respects allows us to take advantage of the variety of highly developed xerographic techniques for exposing the active layer, for developing, transferring and fixing the image, and for wiping the layer of stray charge and ink. For a detailed description of a number of such xerographic techniques applicable to our invention, and of some of the variations and combinations of techniques possible, reference is made to Xerography and Related Processes, edited by John H. Dassauer and Harold E. Clark, London: Focal Press (1965).

It is emphasized in this respect that the arrangement for exposing the pyroelectric layer 19 and the structure of unit 16 in the figure are only illustrative. Direct exposure to an image, such as that employed in conventional photographic devices and processes, is possible with pyroelectrics having sufiiciently high coeflicients. Likewise, numerous structural modfications of unit 16 will be recognized by those skilled in the art. It will be recognized that the exact structure of the unit depends at least in part upon whether positive or negative charge images are desired on the active layer. Thin pyroelectric layer 19 is the only essential portion thereof for the purposes of the invention.

(2) Composition and Fabrication Certain fundamental requirements for the materials of layer 19 of the figure have already been described. It has been indicated that the materials must exhibit primary or secondary pyroelectric properties. Such materials are characterized by the buildup of localized regions of electrostatie charge in response to localized temperature changes. The charge generated by the heating can be positive or negative, depending upon the net dipolar orientation of the material in the layer (i.e., positive pole of the dipolar moment on the exposed or unexposed surface of the layer, respectively).

It has also been indicated that the materials must be capable of fabrication into sheets of relatively large area and small thickness. The large area of the layer allows complete exposure of large images and permits copies of standard sizes to be produced without placing unreasonable restraints on the optics in the system. The small thickness minimizes thermal diffusion of the locally heated regions of the layer which tends to distort the exposed image with time. Thicknesses in the direction of the impinging radiation of the order of a mil are suitable.

Any of a variety or organic polymer materials can be used and are preferred because they are readily available and can be readily fabricated into the required size. The various characteristics of these polymeric pyroelectric materials have been previously discussed in considerable detail in connection with their use for radiation detection. See Vol. 18, Applied Physics Letters, pages 203-205 (March 1971), and Vol. 42, Journal of Applied Physics, pages 5219-5222 (December 1971).

To summarize, it has been found that the polymeric pyroelectric materials typically have a substantial net dipolar moment. The pyroelectric effect in the materials is due to a change in moment in dipoles which have their origin in the symmetries of the crystalline structure, and the magnitude of the effect has been found to depend on the strength of the moment. Since the polymers of concern are made up of chains which are primarily or at least largely carbon, the substituent grouping is so chosen as to have an electronegativity substantially different from that of carbon. A particularly useful bond, therefore, is the carbon-fluorine bond and a preferred class of materials is exemplified by polyvinylidene fluoride (PVF The requirement of net dipolar moment, in turn, requires that there not be total cancellation. Accordingly, totally fluorinated straight chain polymers are generally not useful.

Polymeric pyroelectric materials suitable for the invention are also found to be highly crystalline and are properly classified by space-group designations of the nine classes which correspond to crystalline symmetries which permit the existence of ferroelectricity. PVF is of the point-group designation C Other useful representative materials including polyacrylonitrile, polyvinylfiuoride, poly-o fiuorostyrene and polyvinylidene chloride (all belonging to a polar point group, i.e., C and C where n is equal to 1, 2, 3, 4, or 6). The use of such polymeric films as a direct substitute for PVF in layer 19 is specifically suggested.

Crystallographic alignment is readily achievable during fabrication of the usual polymeric film sections by mechanical working, e.g., by biaxial stressing. Net dipolar orientation or poling, either short-term or continuous, is achievable by the imposition of an electric field, generally a dc electric field, of appropriate strength along the thin dimension of the film. As in most ferroelectrics, increasing temperature permits reduced poling fields. Initial poling is usually carried out with the material heated to near its melting point and with the field generally maintained as the temperature is gradually reduced.

It has been found that suitable pyroelectric behavior may also be obtained in polar materials with a total absence of ferroelectric coupling. Depending upon a variety of factors such as molecular weight, operating temperature, etc., the poling can be frozen-in in suitable materials so that they manifest a remanent polarization and so that the poling field need not be maintained during use.

Conventional inorganic crystalline pyroelectric materials can also be used for layer 19 of the figure, provided they can be fabricated or grown into the desired size and shape. Single crystalline pyroelectric materials such as lithium tantalate, LiTaO for example, are known to be more sensitive per unit area than the typical organic pyroelectrics. Such materials would be useful in arrangements of our invention in which high sensitivity is more important than large exposure and copy area. Inorganic crystalline pyroelectric ceramics, such as lead lanthanum zirconate titanate, PLZT, are also attractive because of their high pyroelectric coefficients and capability of being fabricated in large, thin sheets or films. The use of the foregoing materials in thin sheets as a substitute for PVF layer 19 is also specifically suggested.

(3) Example In an initial experiment, a copier of the type shown in the figure was constructed using a commercially available PVF film which was prepared by biaxial stressing. The film was about 50 percent crystalline as measured by standard density and/or X-ray techniques. Thickness of the film was about 0.8 mils. The area of the film was about 3 cm. x 3 cm. Poling of the film was carried out by the application of an electric field of 1500 volts parallel to its thickness starting at about C., and by cooling to room temperature under the influence of the field. A thin conducting layer (-200 A.) was then deposited on one side of the film by evaporation of aluminum. The ex 7 posed surface of the aluminum was then painted with flat black paint.

The PVF film was found to have a pyroelectric coefficient dP/a'T of approximately 2x10" coul./cm. C. The positive pole of the net dipolar moment was oriented on the unexposed surface of the film so that the heated regions of the unexposed surface developed a negative charge, while the cooler or unheated regions maintained a relative positive charge.

Source 11 of the figure was selected to be a simple 650 watt tungsten-halide lamp. Document 13 was a trans parent slide containing black characters. Lens 12 was a cm. f. 1. lens spaced about 6.35 cm. from both lamp 11 and document 13. Lens 1.5 was a 12.7 cm. f. 1. lens spaced at about 25.4 cm. between document 1.3 and unit 16. During exposure of the PVF; film through the slide for approximately 2 seconds, surface 21 of the film was dusted with negatively charged, dry, powdered ink. The ink was attracted to the cooler regions of the film and the characters became readily visible. The toner image was transferred simply by contacting the exposed and toned surface of the film with a piece of positively charged white paper.

We claim:

1. An electrostatic copying device of the type comprising in combination a source of a beam of radiant energy, means for directing said beam through an object to be copied to produce in said beam an intensity pattern in the pattern of the object, a recording medium having two opposed major surfaces, means for projecting said beam from the object onto one of said major surfaces of said medium to produce thereon an electrostatic charge distribution in the pattern of the object, and means for developing said electrostatic charge distribution on said medium, said device being characterized in that said recording medium comprises a sheet of pyroelectric material having a pyroelectric coefiicient of a magnitude at least as great as coulomb per centimeter squared per degree Centigrade and having a thickness in the direction major surface of said pyroelectric sheet opposite to said beam being set prior to the incidence of said beam on said medium at ground potential, said device being further characterized in that said beam incident on said recording medium is at least partially absorbed by said absorbing layer to cause localized heating of said pyroelectric sheet by direct conduction through said conducting layer, said electrostatic charge distribution in the pattern of the object being generated on the opposite major surface of said sheet pyroelectrically in response to the localized heating thereof accompanying the incidence of said beam.

2. The device of claim 1 in which said sheet consists essentially of a polymeric pyroelectric material having a net dipolar moment in the direction of the sheet thickness.

3. The device of claim 2 in which said sheet is a normally solid pyroelectric polymer selected from the group consisting of polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyvinylfiuoride, and poly-o-fluorostyrene.

4. The device of claim 3 in which said sheet consists essentially of polyvinylidene fluoride.

5. The device of claim 1 in which said sheet consists essentially of a single crystalline pyroelectric material.

6. The device of claim 1 in which said sheet consists essentially of a pyroelectric ceramic material.

7. The device of claim 1 in which said sheet of pyroelectric material has a thickness of the order of one mil.

References Cited UNITED STATES PATENTS 3,713,822 1/1973 Kiess 961 C 3,641,346 2/1972 Lachambre 250-833 H OTHER REFERENCES Journal of Applied Physics, vol. 42, No. 13, December 1971, pp. 5219-5222.

Journal of Applied Physics, vol. 41, No. 11, October 1970, pp. 4455-4459. I

RONALD H. SMITH, Primary Examiner J. L. GOOD-ROW, Assistant Examiner US. Cl. X.R. 

